JPH086273B2 - Pulp processing method and its equipment - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method and apparatus for treating pulp. In particular, it is suitable for treating suspensions in the pulp and paper industry, and more particularly it can make the operation more efficient and reduce the need for equipment, and consequently significantly reduce the investment costs. Can be a pulp mill screening plant.

Wood pulp is manufactured by various methods in the pulp and paper industries. Pulp can be produced chemically by steaming or mechanically by milling and refining. It is also produced from waste paper by pulping with defibration. It is common to all pulp making processes that this pulp contains some impurities that must be removed therefrom.

The pulp is washed in the screening plant with screens and cleaners. This screen is an element in which the pulp is washed by either slotted or perforated screens within a concentration range of 1 to 5%. A cleaner, commonly referred to as a so-called sentry cleaner, is an element that cleans pulp by centrifugal force to a low concentration of usually less than 1%.

However, screening has two problems. First, it is usually desirable after screening to increase the pulp concentration in the range of 10 to 15% for storage or post-treatment. Second, treatment of the reject by purification or some other method is also desired, but usually at higher concentrations than in screening. In other words, the pulp flow must always be thickened after screening.

There have been attempts to solve this problem completely, for example by the Swedish company Kamyr Ab. The solution is aimed at raising the concentration to 8-15% in the screening device. Efforts have been made to develop both a screen and a cleaner that operates at a concentration of about 10%. However, this was only partially successful. Screening and cleaning, as such, are quite successful at high consistency, but at higher concentrations, the separation efficiency between screen and cleaner is significantly reduced. Kamyr allegedly replaced one problem with another, eliminating the need for thickening at the expense of cleaning efficiency.

The present invention includes a method and apparatus for screening pulp in a pressurized enclosed space with a screen in the concentration range of 1 to 5% and with a Sentry cleaner at a concentration of less than 1%. The features of this method and device are described in the claims.

The method and device according to the invention will be described in detail below by way of example with reference to the accompanying drawings.

FIG. 1 is a basic diagram of a screening plant commonly used today. Its structure and operation are described in detail below. Although there are many other screening charts that differ significantly from the details of the flow chart shown in FIG.
The figure shows the basic principles of commonly used screening.
Pumps used in pulp feeding in the accompanying drawings are generally designated by the reference letter P, because the pump itself has no substantial meaning with respect to the invention.

Pulp is fed from the pulp store 1 through knotter 2 and intermediate tank 3 to screens 4-8 at a concentration of about 5%, which screen is divided into two stages in the embodiment of FIG. The second consists of screens 6, 7 and 8. The washed pulp from the last screen is transferred to one or more suction filters 9. In knotter 2, large pieces of knots and the like are separated from the pulp and further knot washer
Transferred to 10, where the acceptable fibrous material is separated from the knot pulp and returned to the intermediate tank 3. This knot material is removed from knot washer 10 and transferred to the next process, knot silo 11 in the illustrated embodiment. Small impurities of binding fibers, fiber bundles and the like are separated from the pulp on screens 4-8. The rejects from screens 4 and 5 are directed to the second screen 12 and thus the accepts from this screen are directed into the screens 6, 7 or 8 of the second screening stage and the rejects to the intermediate tank 13 or directly. It is led to the reject thickener 14, from which it is further led by the feed screw 15 to the purification stage 16. The refined pulp from the refiner and the rejects from the second stage screens 6, 7 and 8 are directed to another second screen 17, the accept of which is directed to the cleaner 18. Acceptance from the cleaner 13 is directed to either the intermediate tank 18 or directly to the first second screen 12. Accepted pulp from the screen enters the suction filter at a predominantly 1 to 2% concentration after screening. The reason is that the suction filter cannot handle pulp with higher inlet concentrations and the pulp is concentrated to a concentration of 10 to 15% by drawing water from it by gravity actuated drop legs. As a result of this inevitability, the pulp mill must have a storey at a height of about 10 m, at least for the suction filter arrangement. The other parts of the device are arranged in various levels according to need and space.

The above screening has some problems. First, when thickening the pulp with filters 9 and 14,
Also, in the transfer of pulp, a lot of air mixes with the pulp and the filtrate, for example, causing foaming. Also, the building height required for the filter is considered a significant drawback. If the screening plant is of a structure that is pressurized to prevent mixing of air and pulp and is hydraulically sealed, all of the above drawbacks would be eliminated.

Second, it requires a great deal of equipment. For example, the screens may be arranged in two successive stages, the first stage including two screens connected in parallel and the second stage including three screens connected in parallel. A very large number of devices is required, since the aim is to have the highest possible separation efficiency in each device. In other words, the desired fraction is attempted to be completely separated from the undesired fraction, ie the ideal is to keep the reject ratio as small as possible. This causes the pulp to circulate in the equipment for a relatively long period of time, thereby using only part of the maximum performance of the equipment.

Thirdly, the low concentration of the suspension to be treated is problematic. Low concentrations by themselves require large filters, ignoring that suction filters are by no means the most efficient type of filter when comparing the operating efficiency of filter surfaces. For example, the concentration of pulp entering the filter is 1.5%, and then 15
It is possible to raise it to%. A total of 15% suspension was used to produce 15 tons of dry fiber pulp through the filter.
You have to take 100 tons from it. As a result of this, 900 tons of liquid must be removed from the 1.5% pulp entering the filter. At a pulp concentration of 3% entering the filter, only 400 tons of liquid need be removed, and at an inlet concentration of 4.5% only 233 tons of liquid are removed. Thus, if the nominal enrichment performance of the filter does not change, doubling the inlet concentration will require only about one-fourth of the enrichment area of the enrichment device required for low enrichment areas. It can be managed by a certain device.

FIG. 2 shows in detail the method according to the preferred embodiment of the invention and the equipment required therefor. Pulp is fed from tank 1 to knotter 2 and further through screen 21 to drum displacement device 23. Screening 1
Occurs in the concentration range from 1 to 5%, usually in the concentration range from 3 to 5%. The drum displacement device 23 is a drum equipped with a cell for thickening the pulp to a consistency of 10 to 15% by the pressure of the inlet pulp. Air does not enter this process. A specific example of the drum displacement device is disclosed in, for example, US Pat. No. 4,502,171.

The reject pulp from the first screen 21 is fed directly to the second screen 22, its accepts are returned to the first screen 21 and the rejects are pressurized and led to a closed thickener 24, from which the pressurized state is reached. Pulp flows to the refiner 25 without a feed screw.
The pulp is fed at a low consistency into thickener 24 and the filtrate is removed during the turbulence effect. Suitable dimensions (diameter 1-
The selection of 2 mm) pores contributes to the major fibers discharged with the filtrate, and then the remaining thickened reject pulp flows further to the refining stage. Thus, refining and thickening of the reject pulp is also done under pressure and air does not mix with the pulp. Element 24 is disclosed in FI patent application 874854. The filtrate from the thickener 24 and the filtrate from the drum displacement element 23 are fed to a cleaner 18 with the pulp to be washed, and the fractions received by these cleaners are directed to the thickener 26, the filtrate also being removed by the cleaner 18. Is supplied to. Thickener 26 is a pressurized water separator disclosed in FI Patent Application No. 873020. Thus and subsequent pulp cleaning and thickening takes place in the pressurized and closed state.

When comparing the device shown in FIG. 2 above with the first device, it can be seen that there are differences in both the number of screens and the method of thickening. When the filtrate is transferred to a small pressurized thickener, the factory layout is compact and, by estimation, the building volume required is less than half that required by conventional screening plants. The reject treatment device according to FIG. 1 comprises a suction filter, a feed screw and the refiner itself. There, the reject process is carried out with a much smaller size thickener and without a feed screw. The number of cleaners remained virtually unchanged. The only addition compared to the device of Figure 1 is a thickener 26 for accepts received from the cleaner. This thickens the suspension from the concentration level required for the cleaner to the level required for screening. The thickening element is placed last in the screening plant. Current systems require two such devices, a suction filter. These are of the size 4 × 8 m, where the drum diameter is 4 m and the drum length is 8 m. In the system according to the invention, only one 3.5 × 5 m thickening device is required.

On the other hand, the operation of the system according to the invention is based on the fact that the screen itself has dimensions and the performance is optimized for maximum performance, which results in a relatively high reject ratio, approximately 20 to 30%. Is. Thus, the accepts from the screen are perfectly clean and suitable for direct delivery to thickeners, eliminating the second screening step. The job of the second screen 22 is to process the reject, or suspension, from the first screen, which still contains a large amount of acceptable fiber fraction, which is returned to the first screen. Thus, in this manner screen 22 corresponds to screen 12 of FIG. 1 processing rejects from the first stage screen.

On the other hand, the operation of the method and device according to the invention is based on the fact that the entire screening is carried out at overpressure so that no external air can enter the process. This is obtained by placing a pump device at a very few key points of the system to pressurize the treated suspension, so that the suspension will be transferred from one device to another by said pressure. . More particularly, this screening plant is divided into a number of pump-screen / cleaner-thickener combinations, where the supply pressure of each pump is sufficient to keep the overpressure, so that on the one hand external gases enter the system. It is not allowed to enter, and on the other hand, the pressure difference required for thickening arises from the pump units of each combination.

The economic advantages mentioned at the beginning of this description are best seen when considering the results of the comparative calculations below.

Energy consumption in screening plants applying the method and apparatus according to the invention is about 34% less than in conventional screening plants.

The cost of buildings and equipment in pulp mills is reference numeral 10
If 0 represents the cost in the conventional screening plant, it is distributed as follows.

Object Conventional plant Pipe of the present invention 100 61 Service platform 100 60 Valves 100 49 Pumps 100 74 Mixers 100 00 Buildings 100 29 Total 100 52 Thus, apart from the energy consumption which is cut down to one-third, electrification, measurement and main Excluding the equipment (including this in the calculation causes too inaccurate and unspecified assessments) the equipment and building costs are estimated to be only about half of the corresponding investment in a traditional pulp mill. See.

In conclusion, the above describes a method of screening and washing pulp in a closed, pressurized space so that the concentration does not have to be increased at the expense of cleaning efficiency. Equivalent method of that the pulp is screened, washed and the rejects are treated in an enclosed space so that the cleaning operation itself is done at the optimum concentration for them and the pulp is still at a higher concentration when it is introduced to the post-treatment stage. Have not been previously disclosed. The method and apparatus of the present invention are not limited to the above embodiments, but they cover all embodiments within the scope of the claims.

[Brief description of drawings]

FIG. 1 is a basic mechanical diagram of a screening plant according to the prior art, and FIG. 2 is a mechanical diagram of a screening plant according to the method and apparatus of the present invention.

Claims (9)

[Claims] 1. Impurities are separated from the defibrated pulp by screening and / or cleaning, and the thus washed pulp is concentrated to a concentration of 10 to 20% and fed to this treatment step for further screening and thickening. Screenable fiber material applicable for depolymerization and defiberization to further treat said impurities in the screening plant and to remove inapplicable impurities from the screening plant after various screening steps, various pulp screening and In the method for treating pulp in a screening plant of a pulp mill, wherein the cleaning step is performed in a closed space of the screening plant or in a pressurized space, the thickening step is performed under pressure at the initial pressure of the screening and / or cleaning step. Characterized by doing, The above pulp treatment method. 2. A method according to claim 1, wherein the concentration of the treated pulp is kept within the range of 2 to 5% during screening. 3. The method according to claim 1, wherein the suspension is thickened after the cleaner at the outlet pressure of the cleaner before introducing the suspension to the screen. 4. The suspension according to claim 1, wherein the suspension from the screen / screen (s) is thickened from a concentration of 2-5% to a concentration of 10-20% at the outlet pressure of the screen / screen (s). the method of. 5. Rejection from the screen / screen (s) / cleaner is directed at the screen / screen (s) / cleaner outlet pressure through a thickener and / or through a screen / cleaner thickening step to a further purification step. The method according to claim 1. 6. The method according to claim 1, wherein all enrichment operations of the screening plant are carried out at overpressure without suction effect. 7. An apparatus for treating pulp in a screening plant of a pulp mill, comprising a plurality of screens, cleaners, thickeners and pumps, wherein all the elements (P, 2,18,21,22,23) of the screening plant. , 24,25,
26) is closed and operates under pressure and the screening plants are each arranged one above the other so as to contain at least one pump (P) -thickener (23,24,26) combination, Pulp processing equipment. 8. The screening plant according to claim 7, comprising at least one closed combination of pump (P) -thickener (24) -fibrizer / refiner (25) operating under pressure. apparatus. 9. A screening plant comprises a large number of pumps (P) -screens (2,21,22) / cleaners (18)-
8. Device according to claim 7, comprising thickeners (23,24,26) combinations, whereby the overpressure required for each thickener (23,24,26) is obtained from the pump unit (P) of each combination.